Lead zirconate titanate is a perovskite in which the A-sites of the perovskite are occupied by bivalent lead (Pb2+) and the B-sites of the perovskite by quadrivalent zirconium (Zr4+) and quadrivalent titanium (Ti4+). To influence an electrical or piezoelectrical characteristic such as permittivity, Curie temperature, coupling factor or piezoelectric charge constant (for example d33 coefficient) PZT doping is used.
A piezoceramic material in the form of a monolithic multilayer actuator is known from EP 0 894 341 B1. The multilayer actuator consists of a plurality of piezo elements arranged one above the other to form a monolithic stack. Each of the piezo elements forms an electrode layer, a further electrode layer and piezoceramic layers arranged between the electrode layers. Piezo elements adjacent to each other in the stack have a common electrode layer in each case. The electrode material of the electrode layers consists of a silver-palladium alloy. The piezoceramic layers have a piezoceramic made of PZT. The nominal empirical formula of the piezoceramic is PB0.99Ag0.01LA0.01 [Zr0.30Ti0.36(Ni1/3Nb2/3)0.34]O3. This PZT with nickel and niobium has a complex B-site doping.
To produce the multilayer actuator, powdered oxidic metal compounds are mixed to form a piezoceramic composition with the formal empirical formula PB0.99LA0.01 [Zr0.30Ti0.36 (Ni1/3Nb2/3)0.34]O3,005. The piezoceramic composition consists of a mixture of powered metal oxides. This mixture is worked in a molding process into ceramic green tapes. The ceramic green tapes are printed with an electrode material made from a silver-palladium alloy with a palladium fraction of around 30% by weight. The printed green tapes are stacked one above the other, debound and sintered. During sintering the piezoceramic layers with the piezoceramic are produced from the green tapes with the piezoceramic composition. The electrode layers (inner electrodes) are produced from the electrode materials printed onto the green tapes. The monolithic multilayer actuator is produced by the common sintering of the piezoceramic layers and the electrode layers (cofiring).
The composition of the piezoceramic of the resulting piezoceramic layers is stoichiometric. The stoichiometry is produced by surplus lead escaping during sintering in the form of lead oxide (PbO). Surplus available heterovalent Lanthanum is compensated for by building in silver of the electrode material at the A-sites of the PZT.
The resulting multilayer actuator has outstandingly good piezoelectric characteristics. For example the Curie temperature Tc amounts to around 170° C. However the sinter temperature to obtain the good piezoelectric characteristics lies at over 1100° C. To avoid the electrode material melting at these high sinter temperatures the fraction of the electrode material by weight of palladium must be at least 30%.
For reasons of cost it is desirable to lower the fraction of palladium in the electrode material or in general to use cheaper electrode material such as copper or silver. These metals each have a melting point of below 1100° C. This means that the sinter temperature must be lowered.
The known piezoceramic composition is not suitable for compaction at a sinter temperature of below 1100° C. At such a low sinter temperature the improved piezoelectric characteristics are not achieved.